This invention relates to protective relaying apparatus of the type which utilizes phase comparison techniques. More particularly, this invention relates to such relaying apparatus wherein the circuitry employed to perform the phase comparison is unsupervised and also generates an echo trip signal.
In prior art protective relaying apparatus, such as that disclosed in United States Application Ser. No. 548,135 and W. E. 46,197, an echo trip signal is generated for transmission to the remote end terminal of a protected line section whereat it aids in effecting remote tripping. In such prior art apparatus, however, the circuitry utilized to generate the echo trip signal dependent upon other circuitry which is supervised or logically inhibited by the output signals of fault and/or overcurrent detectors. Consequently, the echo trip signal is generated only when the supervisory logic permissives related to fault or overcurrent are satisfied.
Under certain operating conditions, where the breakers associated with each end terminal of the protected line section are both open and one is closed into an internal fault, a lack of fault or overcurrent will block the generation of the echo trip signal. Similarly, if both breakers are closed into an internal fault and a zero-feed condition exists at one end terminal, the generation of an echo trip signal will again be blocked. In either instance, the blocking action of the supervised circuitry will prevent breaker tripping due to the lack of an echo trip signal.